In order to comply with expected future regulations on low and high frequency distortions of main ac power lines and electromagnetic interference requirements, it is necessary to improve waveform quality of ac-to-dc converters. In general, there are two approaches to solving this problem. The first approach entails increasing size and reactance value of passive filter elements, i.e. inductors and capacitors, in order to reduce the high frequency content of the ac-to-dc converter waveforms. Disadvantageously, this approach becomes increasingly expensive at higher power levels and creates other side effects for which compensation must be provided, such as high in-rush currents, low range of stability and poor power factor. The second approach entails increasing the switching frequency of the converter waveforms such that filter size can be reduced, since filter size is inversely proportional to the frequency content of the filtered waveform. The disadvantage of this second approach resides in the need for active power devices and additional control circuitry. For many applications, the additional cost would not be compensated for by the lower cost of filter elements and/or other possible technological advantages, such as unity power factor, low in-rush currents and a higher stability range with respect to line impedance. The reason is that the highest attainable switching frequency of pulse-width modulated (PWM) converters is typically too low, e.g. below 10 kHz, at power levels above 3 kW due to high switching losses in the active devices, i.e. hard-switching.
To overcome the problem of active device switching losses, while enabling operation at higher switching frequencies, soft-switching converters have been developed. In general, there are two types of soft-switching, or resonant, converters: zero-voltage switching and zero-current switching. Zero-voltage switching involves switching the active devices when there is zero voltage thereacross. On the other hand, zero-current switching involves switching the active devices when there is zero current therethrough.
An exemplary soft-switching converter is described in U.S. Pat. No. 4,730,242 of D. M. Divan, issued Mar. 8, 1988, which patent is hereby incorporated by reference. Divan describes a high-efficiency power converter which utilizes a resonant dc link (RDCL) comprising an inductor and a capacitor coupled to a dc bus which are caused to oscillate together at a high frequency in order to provide a unidirectional resonant voltage across the dc link. Suitable control ensures that the dc link voltage resonates to zero at least once per cycle, at which time the active devices connected thereacross are switched, thereby avoiding switching losses. However, the advantages obtained in using such a circuit are partially offset by higher voltage stresses applied to the switching devices, necessitating the use of devices with higher voltage ratings. Fortunately, Divan overcame this problem by employing an active clamp in an RDCL to limit peak voltage stresses, as described in U.S. Pat. No. 4,864,483, issued Sept. 5, 1989, which patent is hereby incorporated by reference. A converter according to the latter cited patent is generally referred to as an active clamped RDCL (ACRDCL) converter.
Despite the hereinabove described developments in soft-switching converter technology, such converters are still too costly for many applications, as a result of requiring the use of multiple active devices. Therefore, it is desirable to provide a resonant converter which achieves lossless switching at high frequencies with fewer active devices. Furthermore, soft-switching converters such as those hereinabove described generally operate in a discrete pulse modulation (DPM) mode which may not provide sufficient waveform quality for many applications. Therefore, it is desirable to provide a soft-switching converter which is capable of high-resolution PWM control.